Solar cells and other devices may be created through the use of ion implanters. These ion implanters are used to introduce dopants into portions of the workpiece to create an electrically conductive region.
FIG. 1 shows a block diagram of a conventional ion implanter 100. Of course, many different ion implanters may be used. The conventional ion implanter may comprise an ion source 102 that may be biased by a power supply 101. The system may be controlled by controller 120. The operator communicates with the controller 120 via user interface system 122. The ion source 102 is typically contained in a vacuum chamber known as a source housing (not shown). The ion implanter system 100 may also comprise a series of beam-line components through which ions 10 pass. The series of beam-line components may include, for example, extraction electrodes 104, a 90° magnet analyzer 106, a first deceleration (D1) stage 108, a 70° magnet collimator 110, and a second deceleration (D2) stage 112. Much like a series of optical lenses that manipulate a light beam, the beam-line components can manipulate and focus the ion beam 10 before steering it towards a workpiece or wafer 114, which is disposed on a workpiece support 116.
In operation, a workpiece handling robot (not shown) disposes the workpiece 114 on the workpiece support 116 that can be moved in one or more dimensions (e.g., translate, rotate, and tilt) by an apparatus, sometimes referred to as a “roplat” (not shown). Meanwhile, ions are generated in the ion source 102 and extracted by the extraction electrodes 104. The extracted ions 10 travel in a beam-like state along the beam-line components and implanted on the workpiece 114. After implanting ions is completed, the workpiece handling robot may remove the workpiece 114 from the workpiece support 116 and from the ion implanter 100.
During the ion implant process, the ions typically implant not only the desired face of the workpiece, but also the edges of the work piece 114. The implanted ions at the edge of the workpiece 114 may form a junction, upon activation of the implant. This edge junction may be undesirable, and may cause yield issues, such as shunting of solar cells.
This formation of a junction along the edge of a workpiece 114 may be due to several phenomenons. One possible cause is imperfect ion beam collimation, which may cause the ion beam to diverge more than desired, causing ions to strike unintended areas. Another possible cause is back-sputtering of material from surfaces around and behind the workpiece 114. For example, the material sputtered may be conductive, such as aluminum and create a short circuit from the top surface of the workpiece 114 to the bottom surface. In other embodiments, a non-contaminating material, such as graphite, is used for the material near the workpiece. While graphite does not cause a shunt, it does not help in preventing junctions caused by divergent ion beams. Furthermore, this problem is exacerbated in the case of solar cells, where the workpiece, after being processed, is not cut or divided into smaller portions, as is routinely done with integrated circuits.
Therefore, it would be beneficial if there existed a system and method for implanting ions into a workpiece, and more particularly, a solar cell, without creating shunts along the edge of the workpiece.